A road or highway may be defined as an overland route between two points. To clarify some terminology, a road is generally a narrow route in a rural area; a street is an urban route; and a highway is a wide road that can carry more traffic at higher speeds. In the United States, a major highway--a multilane divided roadway with limited access (relatively infrequent entrances and exits)--is called an expressway, a freeway (if it has no tolls), or a turnpike (if it has tolls). Other names are used elsewhere: motorway (Great Britain), autobahn (Germany), and autostrada (Italy). HISTORY Modern roads are usually thought of as being mainly for wheeled traffic, but the Incas in South America, who never discovered the wheel, had a well-maintained road system stretching from modern Ecuador about 3,680 km (2,300 mi) southward. Even before the Incas, perhaps as early as 3,000 BC, the civilizations of Egypt, Mesopotamia, and the Indus Valley developed roads, first for pack animals and then for wheeled vehicles. The Persians, beginning from the 6th century BC, linked up existing highways to form the "Royal Road" from Ephesus to Susa, and in the 3d century BC the Ch'in dynasty in China established a country-wide network of roads. Roman Empire The greatest road builders of the ancient world were the Romans who, in the three centuries before Christ, made themselves rulers of all the countries bordering the Mediterranean and far beyond. As they developed from a city-state by conquering other small states, they built roads into the conquered regions to help consolidate their gains. They built the Via Latina after they had conquered Latium in 340 BC to connect Rome with Capua. At the instigation of Appius Claudius Caecus, who paid for part of the work himself, the most famous of \TRoman roads\t, the Via Appia (the Appian Way), was built on an alternative route to Capua starting in 312 BC; by 244 BC the road had reached Brundisium on the heel of Italy. By the time of Hannibal's invasion, the Roman road system was well established in central and southern Italy. The revolutionary feature of the Appian Way was that it was paved, partly with stone and partly with lava. After that time road building formed an important function of Roman government, and before the end of the empire, good roads extended from the Forth River to the Euphrates, from the mouth of the Rhine to the Pillars of Hercules, and along the whole length of the North African coast. A remarkable network of roads was also built in the granary of Rome (now Tunisia). A postal service was organized for transmitting government messages. The confidence with which Saint Paul wrote to the early Christians in various parts of the empire indicates that the service was soon used for private letters as well. Most of the road users traveled on horseback or on foot; high officials and the wealthy, however, used two- and four-wheeled carriages (see \Tcoach and carriage\t). Posting stations where horses could be changed and attended to were placed about every 16 km (10 mi) along the roads for government use; inns were also placed at reasonably frequent intervals to provide facilities for private travelers. The Roman achievement is all the more remarkable because the surveyors had to plan the line of a new road and to make it as straight as possible without modern instruments (not even a compass) and often in wild and wooded country. A major reason why their roads lasted was their insistence on adequate drainage. A ditch was excavated a few feet from each side of a road, and the earth from this was used in forming a bank called the agger, which was constructed on foundations of broken stone, bricks, and pottery cemented with lime. Large polygonal blocks of hard stone or of lava (where available) were carefully fitted together to form the top surface of the road. The Latin word for this surface was pavimentum--the root of the modern term pavement. The barbarian tribes, which broke into the Western Empire from the 4th century AD onward and finally destroyed it, were not concerned with centralized power; rather, they preferred the opposite. They disliked cities and in their settlements tended to avoid the Roman roads, which gradually decayed. Centuries passed before any real attempt was made to reconstruct the road system. Middle Ages As national states began to form in Europe, major routes became necessary to provide for the itinerant royal courts to move about. Beginning in the 11th century, the urge to go on pilgrimages to sacred shrines accelerated the development of international trade and made the roads busier than they had been since the collapse of the Roman Empire. Although road surfaces were poor by modern standards, the roads were well provided with inns and smithies. Wheeled vehicles were rare; both goods and people were carried on horseback. Great Britain The development of centralized government was not accompanied by centralized responsibility for road maintenance. In England the responsibility lay with the parishes, which each year elected a highway surveyor who could call on the able-bodied men of the parishes for a specified number of hours annually to repair the roads. Not until 1555 did the surveyors have statutory backing to enforce labor on the roads; in practice, however, the statute was practically unenforceable. Next came the era of the \Tturnpike\t trusts. A number of people would join together to obtain parliamentary powers under which they would take over a length of road for 21 years or build a new one and pay for its maintenance by collecting tolls. Road engineering, however, was still rudimentary, and many trusts did not know how to preserve the roads. Real and lasting improvement came through the work of three engineers. John Metcalf (1717-1810) worked mainly in the north of England and, despite the fact that he was blind from the age of 6, devised a method of carrying roads across marshy ground on rafts of ling or heather tied in bundles. John Loudon McAdam (see \Tmacadam\t), a Scot, became a commissioner for highways. He devoted himself to studying methods of road improvement and, after settling in Bristol, became famous for his success in building roads capable of carrying relatively high-speed wheeled traffic. His method was to put down three layers of small stones, each stone weighing up to about 170 g (6 oz); each layer, about 10 cm (4 in) thick, was first rammed down by hand and then by horse-drawn rollers. First the ramming and then the weight of passing traffic ground the edges off the stones, making a watertight surface. Macadamized, or macadam, roads were almost standard in Britain until the onset of the motor age. The rubber tires of high-speed automobiles tended to loosen the stone surface, adding to the dust raised by vehicles. This problem led to a search for better road surfaces, such as asphalt (see \Ttar, pitch, and asphalt\t). Thomas \TTelford\t was another Scot who gained worldwide fame as an engineer. His method was to make the roads strong enough to take the maximum likely weight of traffic, so he gave more attention to the foundations than McAdam did. Heavy blocks of stone were put down with layers of smaller stones on top of them, and then the road was surfaced with gravel. France To the strongly centralized government of 18th-century France must be given the credit of making the first determined effort to evolve an adequately funded national system of roads. A department of bridges and roads (Corps des Ponts et Chaussees) was set up in 1716, and specifications were laid down for various types of road. Road engineering was recognized as a profession, largely because of the achievements of Pierre Tresaguet, who became inspector general of roads in 1775. His method of construction differed from those of McAdam and Telford because, although he also used layers of stones decreasing in size toward the top, he made each layer slightly cambered (curved from side to side, higher in the middle, and sloping downward toward both edges) for better drainage. The road had a stone curb at each edge to prevent the small stones from rolling off the road. He also made greater use of the subsoil to support the road. Napoleon had French road engineers make great improvements in the roads in many parts of Western Europe regardless of the cost in money or lives. The new roads over the \TSimplon Pass\t and Mont Cenis Pass were outstanding examples. Before Napoleon's time, the divided state of Germany had militated against the provision of more than short lengths of all-weather roads (roads of high enough quality to be passable even in poor weather, such as heavy rain). United States In the United States the old Indian trails served to mark the way for modern roads. The Mohawk Trail and the Natchez Trace are familiar names. Santa Fe, a missionary station receiving supplies from Mexico, gradually became an important trading post and attracted pioneers from the United States. As early as 1825, Congress voted $30,000 to survey and mark a trail to Santa Fe from Independence, Mo. The great westward migrations of the 1840s produced the Overland Trail with its branches to Oregon and eventually to California. The coming of the first \Ttranscontinental railroad\t in the 1860s reduced the importance of the trails, until the development of the internal-combustion engine and automobile gave roads renewed importance. The British general Edward \TBraddock\t in 1755 built a road out from Cumberland, Md. (the Cumberland Road) to assist in his march on the French at Fort Duquesne; little was done, however, to provide the settled states with all-weather roads until, as in England, turnpike trusts were established. Virginia built a turnpike in 1785 with public funds, but private enterprise provided the greater part of the network that developed and served the eastern states before the railroads came. The states were often reluctant to finance the building of roads, and the federal government was even more so, although it did make a start in 1811 by contracting for the construction of the \TNational Road\t, virtually a reconstruction and extension of the Cumberland Road. After many years the National Road reached Saint Louis. After 1838 responsibility for the road was turned over to the individual states. The proliferation of \Lrailroad\ls in the United States in the second half of the 19th century and electric interurbans (high-speed intercity trolley lines) in the East and Midwest from about 1900 to 1920 somewhat reduced the need for all-weather roads even after the automobile had become a practical vehicle. Not until after World War I was a determined attempt made to improve the road system, but the new construction of this period was virtually confined to roads serving expanding communities. The development of special-purpose roads in the United States actually dates from a concept that came from environmental considerations and not from a desire to allow motor traffic to travel without hindrance. William White Niles, a prominent member of the New York Zoological Society, persuaded the city in 1907 to buy sufficient land on both banks of the Bronx River to stop pollution of its water, and, as a part of the scheme, a four-lane road was constructed with limited access and no crossings on the road level. This small beginning led to the widespread system of scenic parkways. During the Depression, Pennsylvania sponsored road improvements among other public works, creating the Pennsylvania Turnpike Commission, which, by the sale of bonds plus a direct state loan, raised enough money to build a 256-km (159-mi) motorway between Harrisburg and Pittsburgh. This toll road, opened in 1940, is generally considered the first superhighway, and many others soon followed. New roads, it was discovered, can generate substantial business in the localities they serve, sometimes, however, with disastrous effects on the environment. In developing the complete highway, American engineers learned that the attainment of high speeds could not be the sole consideration. A road designed to avoid monotony for the driver is a contribution to safety (see \Tsafety, automotive\t). One difficulty facing all highway planners is how to connect highways with central cities without destroying part of the city itself. Tunnels and viaducts both have environmental drawbacks, and clover-leaf junctions with up to four levels of roadway consume a large amount of land. Serious thought about the growth of long-distance road traffic led to a proposal in 1944 for a National System of Interstate and Defense Highways. Under the Federal Aid Highway Act (1956), the Federal Highway Trust Fund was established. Supported by certain federal taxes, it contributes 90% of the cost for roads in the \TInterstate Highway System\t. Canada In Canada, even though roads are a provincial responsibility, the government of the Dominion paid much of the cost of the \TTrans-Canada Highway\t, a 7,820-km (4,859-mi) stretch of highway constructed in 1950-62, and has contributed to the \TAlaska Highway\t and the Mackenzie Highway, which runs north from Edmonton. The Alaska Highway, which runs 2,451 km (1,523 mi) from Dawson Creek, British Columbia, to Fairbanks, Alaska, is maintained by both Canada and the United States. This highway, originally built in 1942 for U.S. military purposes, has contributed enormously to the development of the area. International In 1936 the International Association of Automobile Clubs proposed an international road system for Europe, but World War II stopped any serious discussion. Since World War II, however, individual countries have done much to develop more adequate systems. Schemes were put forward in Britain in the 1920s for special motor roads. Because these were to be built by private bodies that would recoup their expenditure by collecting tolls, however, the plans were not popular. The first advances were made by the Italian and German dictatorships, mainly for military reasons. Mussolini encouraged private firms to construct the autostrada; the first one opened in 1924. Although the pre-1939 roads were poor by today's standards, these early motor roads did incorporate the essential features of limited access and the absence of grade crossings (crossings on the same level as the road). Hitler grasped the value of an adequate road network and, in 1933, Fritz Todd was appointed inspector general of roads with orders to build 4,023 km (2,500 mi) of autobahnen (singular, \TAutobahn\t). By 1942 he had constructed 2,108 km (1,310 mi) of roadway. Tremendous damage was done to the roads during the war, but this damage has been repaired and the roads improved since the war. The Netherlands was the only other country that had a program of highway construction before 1939. Britain set out to build special motor roads (motorways) in 1956 and opened its first in 1959, linking London and Birmingham. Work has continued on a motorway system as well as on projects for improving existing roads and bypassing towns. In 1965, Japan opened its first special motor road--the Meishin Expressway from Kobe to Nagoya (193 km/120 mi), continued in 1970 by the Tomei Expressway from Nagoya to Tokyo (354 km/220 mi). Organizations seeking to improve roads exist in most developed countries. The International Road Federation, which has offices in Washington, D.C., and Geneva, promotes the building of intercontinental highways or the improvement and linking of existing roads. Perhaps the most well-known highway project is the \TPan American Highway\t system, which when complete will link the capitals of all the American nations. North of Panama the traveler can now drive through to Canada, but the Darien Gap on the isthmus of Panama has so far proved to be an impassable obstacle. The I.R.F. also compiles annual statistics on road mileage in all the countries of the world except the \TUSSR\t, East Germany, Kampuchea, Laos, and Vietnam; its figures show the United States with the greatest length of motorways--6,242,000 km (3,879,000 mi). In proportion to area, however, mileage in Belgium (with 127,800 km/79,400 mi) of roads and in the Netherlands (113,600 km/70,500 mi) head the list. ROAD CONSTRUCTION France is generally credited with the first successful laying of pavements of asphalt mastic in the early 19th century. In the middle of the century both France and England used compressed rock asphalt for city streets, and in the United States, bitumen (a natural asphalt) was used. The rapid growth of motoring in the early 20th century encouraged the use of asphalt because it produced a dustproof surface, but all the forms then in existence were apt to become slippery and cause skidding when wet. From about 1929, while a surface was being constructed and the material was still hot, a dressing of bituminous-coated chippings was added, which gave the asphalt road a rough surface. A great asset to the modern highway engineer has been the development of \Taerial photography\t, which greatly assists the preliminary ground surveys for new roads and the planning of routes on the map (see \Tsurveying\t). Before construction of a road begins, estimates are made of the likely cost of possible alternatives, taking into account the geological features, economic needs of the area, and possible damage to the environment. The expensive stage of earth moving must be kept to the minimum, and in areas where a road necessarily has an undulating course, the aim is to balance the cut (the excavations) against the fill (the embankments; see \Tearth-moving machinery\t). Detouring around natural obstacles, such as difficult strata or unstable subsoil conditions, is sometimes preferable to constructing a straight road. Large numbers of laborers built the railroads; in road construction, however, much of the work is done by sophisticated machinery. The process starts with the loosening of the topsoil by a scraper; next, a bulldozer pushes the loosened earth to one side, so that the line of the road is clear. At this point the road is discernible from the air, even though it is nowhere near its final form. Excavations may have to be made, embankments constructed, and sharp curves straightened out. Once the road has its predetermined line and gradients, excavation can begin for the foundations, which must be capable of bearing the expected weight of traffic on the road. Since the mid-century it has become possible in some situations to stabilize the subsoil itself rather than create a foundation of crushed rocks or concrete, provided that the subsoil is fairly uniformly graded. Cement, lime, or bitumen is used as an additive. After the subsoil has been loosened, a binding agent is added, and the mixture is watered, tamped down, rolled, and, when hard, covered with a waterproof surface. Above the foundation, or stabilized soil, comes a base course, usually of concrete, and then the top surface, or pavement, of asphalt or concrete. Shuttering--the use of wooden forms for support--is usually needed to hold concrete in position until it sets, but machines are now available that make this step unnecessary by producing firmly compacted concrete. Concrete can be laid by a spreader that runs on tracks; this step can be followed with the laying of a steel-mesh reinforcement by a crane. Another spreader then puts on the top surface, and a vibrator compacts the surface. Another machine roughens the surface and checks that the camber--which is the slight arch upward in the middle of the road--is proper for good drainage. Charles S. Dunbar Bibliography: Albert, William, The Turnpike Road System, 1663-1840 (1972); Charlesworth, G., A History of British Motorways (1984); Edmonds, G. A., and Howe, J. D., eds., Roads and Resources: Appropriate Technology in Road Construction in Developing Countries (1980); Hindley, Geoffrey, A History of Roads (1977); Hyslop, J., The Inca Road System (1984); Leavitt, Helen, Superhighway--Superhoax (1970); Overman, Michael, Roads, Bridges and Tunnels (1968); Patton, P., Open Road (1986); Seely, B. E., Building the American Highway System: Engineers as Policy Makers (1987); Shank, W. H., Indian Trails to Superhighways (1975); Wright, P H., and Poquette, R. J., Highway Engineering, 5th ed. (1987).